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    Re: lat/long from meridian passage
    From: Jeremy C
    Date: 2011 Jan 27, 00:45 -0500
    Until the Captain yells at you for being 1/2 mile off course without correcting...
     
       In addition, that is about the minimum correction.  If you happen to be heading north when the sun is moving south, you have to make a substantial course change so that you have a vector of the proper size for the correction.  This correction requires you to get very close to E/W or slow down to nearly stopped to get the correct vector to cancel the change in declination.
     
    If you are on a pleasure craft or just going in circles like my ship tends to do, this is perfectly viable solution to simplify the math.  Of course if you are in a sailboat, the wind must allow you to make this heading.
     
     
    In a message dated 1/26/2011 12:48:48 P.M. Central Asia Standard Tim, glapook---.net writes:
    A simple way to deal with the changing declination for determining LAN is to adjust your course to account for the movement of the sun. The sun changes declination fastest around the equinoxes and then it is only at the rate of one knot north or south. Right now the sun is moving north at a rate of 0.2 knots. If your ship, Jeremy, is making 18 knots, even at the equinox you only have to change your course by three degrees so that it will have a vector to the north (or south) of one knot and hold it for a half hour around the estimated time of LAN and this will put you one-half mile off course and add almost zero additional miles to a trans-ocean passage. If you were to do it today you would only have to change course 0.6 degrees to the north which will put you only 0.1 nm off course after a half hour.

    The above applies if your original course was due east or west. On other courses you can calculate the course change  to come up with a north or south component to cancel out the change in declination. But, of course, in this case you also have to make and adjustment to allow for the movement of the vessel's north or south component so the adjustment can be incorporated at the same time.

    gl


    --- On Tue, 1/25/11, Anabasis75---.com <Anabasis75---.com> wrote:

    From: Anabasis75---.com <Anabasis75---.com>
    Subject: [NavList] Re: lat/long from meridian passage
    To: NavList@fer3.com
    Date: Tuesday, January 25, 2011, 6:49 PM

    The idea is pretty basic.  You determine Longitude based on your measured time of LAN.  The trouble is that the when you or the sun are moving N/S while shooting all of the sights, the curve of the apparent altitude is displaced a bit giving you the wrong time of LAN, there therefore longitude.
     
    The method of Lat and Long at transit works best when you are stationary and the sun is near the solstice (changing declination very little).  The method works fairly well with the planets since their change of declination is usually small.  It would work well with stars if you actually observe them over a long twilight.  The moon is probably an exercise in futility without a computer as the declination changes fairly dramatically. 
     
    You can still use the method but corrections for the various bodies' change in declination and your motion are needed.
     
    Jeremy
     
    In a message dated 1/24/2011 10:07:24 P.M. Central Asia Standard Tim, goold{at}vwc.edu writes:
    Jim,
    That is not surprising, since I don't understand the effect of declination change on determination of longitude.  At this point, I am abstracting from as much change as I can.  I am trying to wrap my little brain around the problem of determining the longitude of my front garden.  The problem of establishing the longitude of a vessel in motion is too much at this point.   I would be happy to be instructed, however.  That is the change you are talking about, isn't it-- the observer in motion?

    Patrick

    On Mon, Jan 24, 2011 at 12:23 AM, James N Wilson <jn.wilson{at}juno.com> wrote:
    Patrick:

    You still haven't shown that you understand the effect of declination
    change on the determination of longitude. But you're with the majority.

    Jim Wilson
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